This invention relates to the field of polymer films and, more particularly, to a heat shrinkable biaxially oriented laminated polymer film.
As noted in U.S. Pat. No. 4,194,039, the polyolefins and polyvinyl chlorides can be considered to be the two major families of synthetic resins from which most of the commercially available shrink films for wrapping purposes are manufactured. Other synthetic resins which are useful for the fabrication of shrink films include various ionomers, polyesters, polystyrenes and polyvinylidene chlorides. The shrinkable polyolefins currently on the market are for the most part monolayer films which include both cross-linked and uncross-linked oriented polyethylene, oriented polypropylene, and oriented ethylene-propylene copolymers. The polyvinyl chloride (PVC) shrink films are monolayer films consisting of a variety of formulations of polyvinyl chloride.
As explained in U.S. Pat. No. 4,194,039, a shrink film's distinguishing characteristic is its ability upon exposure to some level of heat to shrink or, if restrained, to create shrink tension within the film. This ability is activated by the packager when the wrapped product is passed through a hot air or hot water shrink tunnel. The resulting shrinkage of the film results in an aesthetically pleasing transparent wrapping which conforms to the contour of the product while providing the usual functions required of packaging materials such as protection of the product from loss of components, pilferage, or damage due to handling and shipment. Typical items wrapped in PVC or polyolefin shrink films are toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
The manufacture of shrink films requires relatively sophisticated equipment including extrusion lines with "racking" capability, irradiation units when cross-linking is desired, tenter frames, mechanical centerfolders, and slitters. "Racking" or "tenter framing" are conventional orientation processes which cause the film to be stretched in the cross or transverse direction and in the longitudinal or machine direction. The films are usually heated to their orientation temperature range which varies with different polymers but is usually above room temperature and below the polymer's melting temperature. After being stretched, the film is rapidly cooled to quench it thus freezing the molecules of the film in their oriented state. Upon heating, the orientation stresses are relaxed and the film will begin to shrink back to its original, unoriented dimension.
For more detailed disclosures of heat shrinkable films, reference may be had to aforesaid U.S. Pat. No. 4,194,039 as well as U.S. Pat. Nos. 3,808,304; 4,188,350; 4,448,792; and 4,390,385.
U.S. Pat. No. 4,377,616, the contents of which are incorporated by reference herein, discloses an opaque biaxially oriented polymer film structure comprising a thermoplastic polymer matrix core layer possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle, and transparent thermoplastic skin layers adhering to the surfaces of the core layer. The unique structure of the core layer imparts a much higher degree of opacity, possibly due to the effects of light scattering, than that possible by the use of opacifying pigment alone. There is no disclosure or suggestion in U.S. Pat. No. 4,377,616, however, of fabricating such core layer and skin layers from heat shrinkable thermoplastic resin.